As a material of optical elements to be used in optical systems of various cameras such as cameras, film integrated type cameras and video cameras, an optical glass or an optical transparent resin is used. Optical glasses are excellent in heat resistance, transparency, size stability, chemical resistance, etc., and there are various materials with different refractive indexes (nD) or Abbe numbers (vD). However, optical glasses have problems of high material costs, bad molding processability and low productivity. In particular, significantly advanced techniques and high costs are required for processing for obtaining an aspherical lens to be used for aberration correction, and this is a major obstacle from a practical viewpoint.
Meanwhile, advantageously, optical lenses made of optical transparent resins, particularly thermoplastic transparent resins can be mass-produced by injection molding, and in addition, an aspherical lens can be easily produced therefrom. Such optical lenses are currently used as camera lenses. Examples thereof include a polycarbonate consisting of bisphenol A, polystyrene, poly-4-methylpentene, a polymethyl methacrylate and an amorphous polyolefin.
However, when using an optical transparent resin as an optical lens, in addition to the refractive index and Abbe number, transparency, heat resistance and low birefringence are also required, and therefore it has a weak point that portions for use thereof are limited depending on the balance of characteristics of the resin. For example, polystyrene has low heat resistance and high birefringence, poly-4-methylpentene has low heat resistance, polymethyl methacrylate has a low glass transition temperature, low heat resistance and a low refractive index, and therefore use areas of these materials are limited, and a polycarbonate consisting of bisphenol A has weak points such as high birefringence, and therefore portions for use thereof are limited. Accordingly, these materials are undesirable.
Meanwhile, in general, when the refractive index of an optical material is high, a lens element having the same refractive index can be realized with a surface having a smaller curvature. Therefore, the amount of aberration generated on this surface can be reduced, and reduction in size and weight of a lens system can be realized by reduction in the number of lenses, reduction in the eccentricity sensitivity of the lens and reduction in the thickness of the lens. For this reason, it is useful to provide a high refractive index.
Further, regarding the optical design of optical units, it is known that chromatic aberration is corrected by combined use of a plurality of lenses with different Abbe numbers. For example, chromatic aberration is corrected by combined use of a lens made of an alicyclic polyolefin resin having an Abbe number of 45 to 60 and a lens made of a polycarbonate resin consisting of bisphenol A having a low Abbe number (nD=1.59, vD=29).
Among optical transparent resins which have been put to practical use in applications for optical lenses, examples of those having a high Abbe number include polymethyl methacrylate (PMMA) and cycloolefin polymer. In particular, cycloolefin polymer has excellent heat resistance and mechanical characteristics and therefore has been widely used in applications for optical lenses.
Examples of resins having a low Abbe number include polyester and polycarbonate. For example, the resin described in Patent Document 1 is characterized in that it has a high refractive index and a low Abbe number.
There is a difference between the hygroscopic expansion coefficient of cycloolefin polymer having a high Abbe number and the hygroscopic expansion coefficient of a polycarbonate resin that is a polymer having a low Abbe number. When forming a lens unit by combining these two types of lenses, the difference between the sizes of the lenses is made at the time of water absorption depending on an operating environment of a smartphone or the like. Due to the difference of the expansion rate, performance of the lens is impaired.
Patent Documents 2 to 4 describe polycarbonate copolymers containing a perhydroxy dimethanonaphthalene skeleton, but since the dihydroxymethyl group is positioned at 2,3-position in each case, the strength is low, and therefore these materials are not suitable for use in optical lenses. In addition, the polycarbonates described in Patent Documents 2 to 4 have a low glass transition temperature (Tg), and accordingly have a problem in terms of heat resistance. For example, the homogeneous polycarbonate described in Example 1 of Patent Document 4 has a number-average molecular weight of 38000, but the glass transition temperature (Tg) thereof is low (125° C.).